Hallo,
the Lu-177 decay data in FLUKA is actually correct, as indicated by the resulting spectrum (thanks to @pschoofs)
where the peaks are photons and the background, extending towards the ~500 keV endpoint, is due to electrons. Note that the energy you took above is in fact the endpoint of the ~80% emission, which is not at all representative of the spectrum, being three times higher than the respective mean beta energy.